A plasticizer is a polymer additive that serves to increase the polymer's flexibility, elongation, or ease of processing. It is typically added during the compounding process of the polymer and interacts with the polymer only physically. The most commonly measured physical effects of a plasticizer include melt viscosity, elastic modulus, and glass transition.
One common class of plasticizers is based on phthalates. Phthalate plasticizers, such as dioctyl phthalate (DOP) and di-2-ethylhexyl phthalate (DEHP), are widely used in polyvinyl chloride (PVC) products for medical delivery systems, children's toys, baby devices, and shrink film.
Shrink film or heat-shrink film is a polymer film that shrinks in one or more directions when heat is applied. To give the polymer film its shrinkability, the film is stretched when it is above Tg of the polymer to orient the molecules from their initial random pattern. Cooling the film sets its characteristics until it is reheated. Reheating causes the film to shrink back towards its initial dimensions.
Unlike PVC shrink film, phthalate plasticizers do not work in polyester shrink film mainly due to compatibility and stability issues. Certain copolyesters, without any plasticizer, can be converted into shrink film, but they suffer from two main disadvantages. One disadvantage is that a copolyester film without a plasticizer has a high shrink on-set temperature. Because of the high shrink on-set temperature, the film will not finish shrinking completely in a steam tunnel, especially on a high-speed line in which the dwell time in the tunnel is short. The other disadvantage is that such a film has a high shrink force. If used as a container label, the high shrink force can cause wrinkles in the film as well as distort the container's shape. Two US patents have addressed these disadvantages.
U.S. Pat. No. 5,589,126 claims a polyester shrink film with 1 to 10 weight percent of a plasticizer selected from a C4 to C20 alkyl ester of an epoxidized fatty acid having 12 to 20 carbon atoms. In one embodiment, the plasticizer is selected from the group consisting of octyl epoxy soyate, epoxy tallates, epoxidized soybean oil, epoxidized linseed oil, triphenyl phosphate, neopentyl glycol dibenzoate, glycerine, vegetable oil, and mineral oil.
U.S. Pat. No. 5,824,398 claims a polyester shrink film containing 1 to 10 weight percent of a plasticizer selected from a C5 to C35 monoglyceride prepared from the reaction of glycerol and a fatty acid having 4 to 30 carbon atoms.
These two patents address the plasticizer compatibility issue, but do not entirely solve the stability issue. In other words, the plasticizer tends to separate from the polymer matrix and migrate to the surface of a film, especially at higher processing temperatures. This is also a prevailing problem for plasticized PVC.
Another (U.S. Pat. No. 6,362,306) discloses a polyester designed to have optimal shrink on-set temperature and ultimate shrinkage to satisfy full-body labeling and steam tunnel operation. Generally, polymers having a high diethylene glycol (DEG) content, may result in a more brittle film which may limit high-speed tentering and down-gauging.
In view of the above, there is a need in the art for polyester-based shrink films that do not suffer from compatibility and stability issues with plasticizers and that do not require high DEG content.
The present invention aims to address this need as well as others, which will become apparent from the following description and the appended claims.